Base mesh becomes unsculptable
You definitely need to have a higher resolution mesh to sculpt. If you have not applied the skin modifier yet, you need to do so. You should save a copy of your .blend file before this. Then you need to subdivide your main mesh, as the skin modifier is a very very low quality mesh creator, when not subdivided. After all that, you should be able to sculpt the mesh. If you still cannot, then please post an image so I can see what settings you have for the mesh. A .blend file would also be helpful. 
If this does not answer the question, please say so in the comments below.